Nowadays, an influence of infrared/ultraviolet radiation is tried to be reduced as much as possible from a viewpoint of energy saving as typified by a reduction of air-conditioning cooling load. From such a tendency, recently, a glass having a structure in which a conductive film is formed on a surface of the glass or a laminated glass having a structure in which a transparent conductive film is sandwiched between adhesive surfaces of the laminated glass comes to be adopted as a window glass for a vehicle in order to reduce a solar-radiation energy which passes into a vehicle interior.
On the other hand, recently, various systems using wireless technologies come to be mounted in the vehicle, such as an AM/FM broadcasting, a terrestrial digital broadcasting, a satellite digital radiobroadcasting, a car navigation system, a keyless entry system, a TPMS (Tire Pressure Monitoring System), an ETC (Electric Toll Collection), a cellular phone, a mobile WiMAX (Worldwide Interoperability for Microwave Access: high-speed wireless network system for mobiles), and a wireless LAN for automobile.
A transmitting-and-receiving antenna(s) for these various vehicle-mounted systems using wireless technologies is provided in the interior of automobile, in many cases. Hence, in the case that the conductive film is formed on a substantially-entire surface of the window glass as mentioned above, there is a problem that airwaves are blocked by the conductive film so that a transmitting-and-receiving performance of the transmitting-and-receiving antenna is reduced severely.
Therefore, various countermeasures are being proposed in order to enhance the transmitting-and-receiving performance in the case that the antenna is provided to the vehicle window glass to which the conductive film is provided.
For example, Japanese Patent Application Publication No. 2001-127520 proposes a method of providing an antenna line in a film-removed portion of a conductive film formed on a substantially-entire surface of a vehicle window glass. (see Patent Literature 1)
Moreover, Japanese Patent Application Publication No. 2001-185928 proposes a method of forming a slot antenna. In this technique, a part of a conductive film formed on a substantially-entire surface of vehicle window glass is cut to form a slot whose size is adjusted in conformity to a desired reception frequency, and then, power-feeding points are provided for the slot. (see Patent Literature 2)
Furthermore, Japanese Patent Application Publication No. 2002-290145 and Japanese Patent Application Publication No. 2005-12587 propose another method of forming a slot antenna. In this technique, a region in which no conductive film is formed is provided to have a certain width along an outer circumference of a vehicle window glass, and thereby, a slot is formed between an outer circumferential portion of the conductive film and a circumferential portion of a flange into which the vehicle window glass is attached. Then, power-feeding points are provided to straddle a region between the flange and the conductive film. In this method, a short-circuiting terminal for electrically short-circuiting the flange and the conductive film is provided between the flange and the conductive film, and thereby, an impedance of the slot antenna is adjusted to be matched with a characteristic impedance of a feeder cable at a desired frequency. (see Patent Literatures 3 and 4)